1. Field of the Invention
The present invention relates to an automatic air conditioning system with a variable displacement compressor, for automotive vehicles, particularly to a system which is capable of maintaining a high displacement of refrigerant therein when quick cooling is required; for example, during summer days in which the ambient temperature is particularly high. High displacement, optimum cooling power, must be maintained even though the suction pressure of the compressor is lowered in response to a decrease in the intake air temperature (as measured just behind the evaporator of the system) to prevent freezing of the evaporator.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various automatic air conditioning systems with a control unit which controls opening angle of doors, such as a fresh/recirculation switching air intake door, an air mixing door, a defroster door, a chest vent door, a foot vent door, or the like, and controls amount of air flowing through the evaporator of the air conditioning system in response to output signals from various sensors for detecting various physical quantities, such as an ambient temperature, room temperature in the vehicular cabin, magnitude of insolation, intake air temperature of the evaporator, suction pressure of the compressor, and in response to output signals indicative of ON/OFF state of various switches, such as an air conditioner switch, a blower switch, an ignition switch, a defroster switch, or so forth. In general, such an automatic air conditioning system controls the intake air temperature of air flowing through an evaporator to prevent the evaporator from freezing. Conventionally, the intake air temperature is measured just behind the evaporator. As is well known, a suction pressure of a compressor is correlatively lowered in accordance with the lowering of the intake air temperature of the evaporator. Therefore, the automatic air conditioning system conventionally controls the above mentioned suction pressure of the compressor in such a manner that the suction pressure is kept higher than a preset pressure which is determined on the basis of a threshold value at which the evaporator starts to freeze. The threshold value of the intake air temperature will be hereinafter referred to as a "freezing start possible temperature". In prior art air conditioning systems, to avoid freezing of the evaporator, that is, to prevent the suction pressure of the compressor from becoming lower than a preset pressure, the air conditioning system controls the suction pressure in such a manner that, if the suction pressure of the compressor becomes lower than the preset pressure, the discharge from the compressor is lowered and thus the cooling power of the system is lowered.
In such air conditioning systems for automotive vehicles, quick cooling is required when the room temperature of the vehicular cabin becomes excessively high, for example in the summer daytime. Under this condition, in conventional air conditioning systems, discharge from a compressor is increased by manually operating a control switch mounted on a controller of the system. One such automatic air conditioning system has been disclosed in the Japanese Utility Model First Application (Jikkai Showa) 60-22418.
However, in such conventional air conditioning systems for automotive vehicles, even if it is set for quick cooling, the air conditioning system controls the discharge of the compressor to lower level such that the suction pressure of the compressor is kept higher than the preset pressure, thereby preventing the evaporator from freezing. However, cooling power becomes lowered with the result that a cooling time required for cooling to a desired temperature within the vehicular cabin becomes long. Furthermore, in such quick cooling, the torque loss is large according to the long cooling time.